1. Field of the Invention
This invention relates generally to wireless communication devices, and more particularly to portable subscriber stations operable between two or more divergent wireless communication systems.
2. Description of Related Art
The modern analog cellular system for mobile wireless duplex voice transmission is called xe2x80x9cAdvanced Mobile Phone Servicexe2x80x9d (AMPS). The AMPS cellular network uses the FCC assigned carrier frequency range of 800 to 900 MHZ. Automobile-mounted cellular units transmit voice signals to a cellular base station within a given cell using up to one watt of power. Hand-held cellular units using battery power supplies transmit voice signals to a cellular base station within a given cell using up to one quarter watt of transmission power.
The AMPS system was designed to communicate the analog human voice signal. The AMPS system was optimized for carrying as many analog voice signals within a given bandwidth of a channel as possible. Mobility of the cellular telephone using low power mobile units, FM modulation, and a higher carrier frequency range (800 MHZ-900 MHZ) is achieved through a cellular arrangement of base stations whereby a user""s signal is handed off to the next cell site as the user moves into a new cell. This cellular hand-off can cause a temporary loss in transmission or reception. However, temporarily losing a voice signal is not critical because a user can easily detect a loss of voice signals and the voice information can be readily retransmitted upon request. However, signal loss, even though temporary, poses special problems for the transmission of digital data. Other examples of inherent AMPS shortcomings which can cause loss in voice signals are diminished signal strength, reflections, Rayleigh fading, and cellular dead spots.
The availability of portable computers has given rise to the desire to conduct wireless transmission of digital data from a remote location. Presently, the AMPS voice cellular system is being used to transmit digital data in the form of circuit-switched cellular data across AMPS carrier channels. Raw (baseband) digital data must first be converted so that it can be transmitted and received using the analog AMPS system. The baud rates available using an AMPS system disadvantageously is, limited due to inherently narrow channel bandwidths and transmission errors.
Heretofore, providing efficient wireless communication of both voice and data signals in an integrated package has been difficult. It has been difficult to integrate the features of AMPS voice transmission with applications such as data transmission, electronic mail, duplex paging, as well as the provision of a circuit-switched cellular data interface such as a wireless fax-modem, into a single hand-held battery operated wireless unit. Some degree of integration of these functions has been facilitated by the development of the Cellular Digital Packet Data (CDPD) system described in the CDPD specification, Version 1.1, hereinafter referred to as the xe2x80x9cCDPD Specificationxe2x80x9d. The CDPD communication system shares the same carrier frequencies assigned to the AMPS channels as described in Part 405, Version 1.1 of the CDPD specification. The base unit or mobile data base station (MDBS 1, as illustrated in FIG. 1), of a CDPD system utilizes a channel within an AMPS cell to establish a link and to communicate with a user""s mobile end system (M-ES 2). The MDBS 1 may use frequencies outside the scope of the AMPS frequencies. The M-ES 2 may be a portable computer, hand-set or some other portable electronic device containing a communication unit. The MDBS 1 serves as a communication link between the user of the M-ES 2 and a service provider""s network of wire lines, microwave links, satellite links, AMPS cellular links, and other CDPD links. For example, the MDBS 1 facilitates communication between the M-ES 2 and a mobile data intermediate system (MD-IS) 3, intermediate systems 4, 5, 6 to convey data to another mobile end system, computer network, or non-mobile or fixed end-user system (e.g. F-ES 7, 8).
As described in the CDPD specification, the CDPD network is designated to operate as an extension and overlay of existing communication networks, such as AMPS networks and the Internet. From the mobile subscriber""s perspective, the CDPD network is a wireless mobile extension of traditional networks. The CDPD network shares the transmission facilities of existing AMPS networks and provides a non-intrusive, packet-switched data service that does not impact AMPS service. In effect, the CDPD network is entirely transparent to the AMPS network.
As defined in the CDPD specification, the CDPD system employs connectionless network services (CLNS) in which the network routes each data packet individually based on the destination address carried in the packet and knowledge of current network topology. The packetized nature of the data transmissions from an M-ES allows many CDPD users to share a common channel, accessing the channel only when they have data to send and otherwise leaving it available to other CDPD users. The multiple access nature of the system makes it possible to provide substantial CDPD coverage simultaneously to many users with the installation of only one CDPD station in a given sector (transmitting range and area of a standard AMPS base station transceiver).
The airlink interface portion of the CDPD network comprises a set of cells. A cell is defined by the geographical boundaries within the RF transmission range from a fixed transmission site such as MDBS 1, which can be received at acceptable levels of signal strength by mobile subscribers such as M-ES 2. The transmitter supporting the cell may be located centrally within the cell, with transmission being carried out via an omni-directional antenna, or the transmitter located at the edge of a cell and transmitted via a directional antenna to cover just a portion of the cell. This portion of the second type of cell is referred to as a sector. In typical configurations, the transmitters for several sectors are co-located. The area served by a set of cells has some area overlap so that a roaming mobile end system can maintain continuous service by switching from one cell to an adjacent cell in a manner roughly analogous to the standard hand-off in the AMPS system. The two cells are considered to be adjacent if an M-ES can maintain continuous service by switching from one cell to the other. This switching process, called cell transfer, is done independently of normal AMPS hand-off procedures.
In FIG. 1 the interface (A) between the M-ES 2 and the MDBS 1 is an xe2x80x9cair interfacexe2x80x9d constituted by a radio frequency link using standard AMPS frequencies. The MDBS 1 is connected to other mobile data base stations through a mobile data intermediate system (MD-IS) 3. A plurality of mobile data base stations can be controlled by a single MD-IS. The MD-ISs are connected to each other through intermediate systems such as system 4 and system 5 in FIG. 1.
The intermediate systems comprise at least one node connected to more than one sub-network (e.g., MD-IS 3). The intermediate system has a primary role of forwarding data from one sub-network to another. Specifically, the MD-IS 3 performs data packet routing based on knowledge of the current location of each mobile end system within the range of the mobile data base stations under the control of the MD-IS. The MD-IS is the only network entity that has knowledge of the location of any of the mobile end systems. However, under some circumstances (as defined in the CDPD specification) particular mobile data base stations track the behavior of specific subscriber stations. A CDPD-specific Mobile Network Location Protocol (MNLP) is operated between each MD-IS (through the intermediate system) to exchange location information regarding the mobile end systems.
The overall CDPD network is controlled by a network management system (NMS) 10 having an interface with at least one mobile data intermediate system 3. Using a special protocol programming instructions can be transmitted from the NMS 10 through the MD-IS 3 to any number of mobile data base stations under proper conditions.
Such programming instructions can be used to convey useful network data to the MDBS, as well as configuring the operation of an MDBS with respect to such critical features as maintaining channel queues. The NMS also controls other CDPD system characteristics such as the timing of paging messages to coincide with the non-dormant periods of the M-ES hand-sets. One advantage of the subject CDPD system is the capability of providing operating instructions to mobile data base stations from the NMS 10 through an MD-IS 3, or by a direct connection to the MDBS as is outlined in the detailed description of the MDBS architecture found in the CDPD specification, Parts 402 and 403.
FIG. 2 depicts a comparison between the CDPD network illustrated in FIG. 1 and the standard AMPS network. The MDBS 1 is the CDPD equivalent to an AMPS base station 21. Both serve as links to mobile users, 2, 2xe2x80x2, and 2xe2x80x3 for the CDPD system and 22, 22xe2x80x2 and 22xe2x80x3 for AMPS users. Both AMPS and CDPD functions can be handled by the same hand-set or end system equipment. Also, the MDBS 1 is preferably located with the AMPS base station 21 as is described hereinbelow in greater detail.
The MD-IS 3 acts as a local controller of the CDPD mobile data base stations which are connected thereto. The MD-IS 3 is analogous to the mobile telephone switch office (MTSO) 23 which is used to control a plurality of AMPS base stations 21, 21xe2x80x2 and 21xe2x80x3. In the AMPS system, the MTSO 23 can be connected to the various base stations 21, 21xe2x80x2, 21xe2x80x3 by way of communication links, either using dedicated landlines or through a public switched telephone network (PSTN). The connection between the MD-IS 3 and the various mobile data base stations 1, 1xe2x80x2, 1xe2x80x3 controlled thereby is made in the same manner. However, different signaling protocols are used than those found in the AMPS system.
In contrast to the infrastructure requirements of AMPS, the infrastructure requirements of CDPD are very small. The CDPD base station equipment is preferably located at a cellular carrier""s cell site along side existing AMPS base station cellular equipment. The multiple access nature of the CDPD system makes it possible to provide substantial CDPD coverage to many users simultaneously with the installation of only one CDPD radio in a given sector. This multiple access capability is the result of a mobile end-system accessing the CDPD channel only when it has data to transmit.
Referring again to FIG. 2, the AMPS base station and the MDBS can use the same RF equipment if both are co-located. By contrast, the MTSO of the AMPS system and the MD-IS of the CDPD system do not have to be co-located to share RF links. In the AMPS system, the MTSO 23 connects AMPS base stations with a mobile station to another party through a PSTN 24. The intermediate system 4 is to a CDPD system as the PSTN 24 is to an AMPS system. Similar to the AMPS system, the CDPD system must use the public switch telephone network or another landline network to complete calls to remote parties or systems via a phone system terminal network 28. However, the CDPD system uses a different protocol than that used in the AMPS system to complete calls over a PSTN.
The MDBS 1 maintains zero or more (up to the MDBS transmission capability) channel streams across the airlink interface, as directed by the MD-IS which controls the MDBS 1. The MDBS 1 instructs all subscriber units or M-ESs (e.g. 2, 2xe2x80x2, 2xe2x80x3) to change channels when necessary. For example, the MDBS 1 instructs M-ES 2 to change channels when an AMPS communication is detected on the CDPD channel. Each subscriber unit""s terminal stream is carried on one channel stream at a time, normally selected by the mobile subscriber, preferably based upon data received from the MDBS regarding optimum channels for CDPD use. The forward and reverse traffic in a given cell (the terminal stream of the MDBS) is carried over a single DSO trunk, between the MDBS and the MD-IS. Communication between the MDBS and the MD-IS over the DSO trunk uses standard digital transmission formats such as the T1 1.544 Mb/s digital transmission standard.
Digital data is transmitted between the MDBS and the M-ES using Gaussian Minimum Shift Keying (GMSK) modulation. Transmissions from the base station to the M-ES are continuous. Transmissions from an M-ES to an MDBS are sent using a burst mode in which each M-ES accesses a channel only when it has data to send and the channel is not presently being used by other M-ESs. This feature allows multiple mobile subscriber stations or M-ESs to share a single channel. The burst mode of transmission also reduces the M-ES connection time as compared to the connection time required when sending digital data using conventional circuit-switched cellular modems.
Unlike the signaling schemes used by conventional cellular modems, which have been chosen based on the need to operate within the constraints of the existing voice signaling system, the GMSK modulation technique used for CDPD communication was explicitly selected with the intent of obtaining both very high bit transmission rates and very good error performance in cellular channels. The fact that the choice of modulation was not constrained by a pre-existing signal structure allows CDPD systems to achieve substantially greater instantaneous bit rates at very low received signal levels when compared to those of conventional cellular modems. This means that CDPD communication systems provide reliable, high speed data transmission in areas where signal quality is inadequate for acceptable cellular modem performance.
The raw (baseband) digital data presently transferred across the CDPD network includes electronic mail messages, digital fax data, and digital data representing a network connection such that files may be transferred as if currently connected to a local area network.
The mobile data intermediate system MD-IS 3 processes the routing of packets for all mobile end systems presently in its serving area. Two services are performed by the MD-IS: a registration service maintaining an information base of each M-ES currently registered in a particular serving location; and a re-address service, decapsulating forwarded packets and routing them to the correct cell. The serving MD-IS also administers authentication, authorization and accounting services for the network support service applications.
In a first mode of operation, a CDPD communication system can operate using dedicated channels which are set aside from a pool of cellular voice channels and reserved for CDPD use. In a second mode of operation, a CDPD communication system can use idle time on channels that are also used by the AMPS system. In this second mode, the mobile data base station can perform xe2x80x9cRF sniffingxe2x80x9d to determine which channels are available and to detect the onset of voice traffic on the channel currently being used for CDPD communication. If an AMPS cellular unit begins transmitting on a channel occupied by a CDPD transmission, the CDPD unit ceases transmitting on that channel and switches to another available channel (a process called xe2x80x9cchannel hoppingxe2x80x9d). If no other channel is available, the CDPD unit ceases transmission until a channel becomes available for CDPD use.
Although the CDPD system shares existing AMPS radio frequency channels, AMPS calls are given the highest priority. AMPS calls are always able to preempt the use of any channel presently being used by CDPD. However, the cellular service provider may opt to dedicate one or more channels for CDPD usage. In this case, AMPS calls will never attempt to pre-empt the channels dedicated to CDPD use.
Typically, the MDBS monitors the activity on AMPS channels during channel hopping. The MDBS maintains a status list (i.e., channels are used for voice or are unused) occupied by voice or unused) for each channel. This status list is available to the CDPD system. The MDBS selects a channel for CDPD use from the unused channels in the list based on a combination of criteria. The criteria can comprise the following: the likelihood that the channel will be required by the voice system in the near future; the interference present on the channel; the interference that the CDPD communication is likely to cause to other voice users in different cells (or on other sectors), and other factors.
The MDBS transmits a list of all the channels which are available for CDPD use to the M-ES""s, whether or not currently occupied by voice communication. This list is used by the MDBS when executing a channel hop before the channel is pre-empted by AMPS communication if the MDBS determines that another channel is more suitable. In this case the MDBS sends a message to the M-ESs directing the M-ESs to hop to the specific channel selected. The MDBS then executes a channel hop. Such a channel hop is much more orderly and efficient than an xe2x80x9cunplannedxe2x80x9d hop because the M-ESs are not required to search for the next channel.
Whenever a CDPD channel is pre-empted by an AMPS communication the MDBS selects another channel from those which are not being used by the AMPS system. The MDBS then immediately hops to the selected unused channel without informing the M-ES (an unplanned hop). The M-ES then determines that the CDPD signal is no longer present on the current channel and searches the other channels in the list to determine the channel (if any) to which the CDPD communication has hopped.
The CDPD system can easily interface to and share equipment with the existing AMPS system. The MDBS must therefore have the ability of physically interfacing with existing AMPS base stations. Consequently, the MDBS is small, non-obtrusive, and easily accessible. The MDBS may be connected easily to equipment which is external to the MDBS which is normally shared with the AMPS system. This external equipment includes an antenna system, RF power amplifiers (in particular, linear amplifiers can be shared with existing AMPS), RF multicouplers, power splitters, duplexers, and other optional AMPS base station equipment. Because the MDBS shares the environment of the AMPS base station, the MDBS must not create a substantial additional burden upon such support systems as environmental control and maintenance. Thus, the MDBS is typically compact and flexible, comprising only those elements necessary for carrying out the CDPD MDBS functions necessary at that cell site.
One of the more demanding technical requirements of the CDPD system is that the ME-S must concurrently monitor the status of both the CDPD and the AMPS communication systems. If the ME-S adheres to the timing of the CDPD system it is possible that some incoming AMPS calls will be missed. This is true, despite the preemptive priority given to AMPS calls versus CDPD communications. And while priority is given to monitoring the AMPS calls, it is probable that a CDPD communication directed to an ME-S will be lost despite the ability of the CDPD system to buffer incoming paging signals for xe2x80x9csleepingxe2x80x9d M-ESs. Existing CDPD and AMPS communication systems fail to provide efficient monitoring of both modes of communications to prevent loss of incoming calls.
Accordingly, it is desirable to provide an M-ES which can concurrently monitor communication activities in both the CDPD and AMPS communication systems. The present invention provides such an improved M-ES. Furthermore, it is desirable to automatically switch user displays depending upon the type of communication system with which the user is communicating. The present invention provides an M-ES which has such an automatic display switching capability.
One advantage of the invention resides in facilitating efficient switching between data communication and voice communication without loss of data communication where voice communication has priority.
The present inventive M-ES provides the advantage of reducing the risk of losing AMPS and CDPD transmissions. Another advantage of the present invention is in efficiently performing a hand-off operation of a wireless subscriber station in a CDPD communication system without losing incoming CDPD or AMPS calls.
These and other advantages of the present invention are achieved by the present inventive M-ES which is arranged for communication with an analog cellular voice communication system and a CDPD communication system, where the M-ES is configured to appear to the CDPD system as if the M-ES is operating in the CDPD mode when in reality M-ES is actually operating in the AMPS mode of communication. In effect, the present inventive M-ES xe2x80x9cfoolsxe2x80x9d the CDPD system into registering the ME-S as operating in a CDPD xe2x80x9csleepxe2x80x9d mode when, in fact, the M-ES is conducting activities on the AMPS system (e.g., receiving AMPS messages). Thus, using the present inventive method, an M-ES will not be de-registered from the CDPD system when operating in an AMPS mode of communication.
In accordance with one aspect of the present invention, a subscriber station is arranged for communication with a first communication system and a CDPD communication system, where the CDPD system includes a first time adjustment means for selecting a first time interval between consecutive CDPD paging messages which are sent from the CDPD communication system to the wireless subscriber station. The wireless subscriber station or M-ES includes means for requesting communication over the first communication system and means for requesting communication over the CDPD communication system. The subscriber station also has a second time adjusting means for selecting a second time interval starting at a most recent CDPD communication and ending when the wireless subscriber station is expected to enter a CDPD xe2x80x9csleepxe2x80x9d mode of operation. The subscriber station also includes means for synchronizing the first and second time intervals to determine the respective CDPD and first communication system operation schedules. The wireless subscriber station uses a means for selecting between operation in the first communication system during the second time interval.
In accordance with the present inventive method and apparatus, the subscriber station operates in the CDPD system as if it is in a CDPD xe2x80x9csleepxe2x80x9d mode while in reality the subscriber station operates in an AMPS mode. The inventive subscriber system achieves this advantageous operation by switching between a CDPD mode and an AMPS mode of operation based upon the AMPS paging cycle and the CDPD TEI notification cycle.
This operation is facilitated by a wireless subscriber station arranged for communication with a first communication system and a CDPD communication system where the CDPD communication system includes a first timer means for measuring a first time interval which specifies the time interval between consecutive CDPD paging messages sent from the CDPD communication system to the wireless subscribe station.
The CDPD communication system also includes a second timer means for measuring a second time interval which specifies the time interval between a CDPD system response to a polling signal from the subscriber station and expected entry of the subscriber station into a CDPD xe2x80x9csleepxe2x80x9d mode. The subscriber station includes means for requesting communication on the first communication system and on the CDPD communication system, means for measuring the first and the second time intervals, and means for synchronizing the duration of the first and the second time intervals with the CDPD communication system. The wireless subscriber station also includes a means for determining respective CDPD communication and the first communication operation schedules for the subscriber station based upon the first and second time intervals and a paging cycle of the first communication system. Also included are means for selecting operation of one or the other of the means for requesting communication based upon the operating schedules to scan for incoming paging signals on the first communication system while remaining registered on the CDPD system.
In accord with a further aspect of the invention, a method for communicating between a wireless subscriber station and both an analog cellular voice communication system (e.g., AMPS) and a CDPD communication system facilitates a subscriber station configured to monitor both incoming analog cellular voice communications and incoming CDPD communications. The inventive method includes the steps of registering the subscriber station with an analog cellular voice communication system and a CDPD system. The CDPD registration process includes the step of synchronizing a first time interval between the subscriber station and the CDPD system where the first time interval defines when the subscriber station is expected to next be on the CDPD channel. In a next step of the CDPD registration process, the subscriber station is switched from the CDPD channel to an analog cellular voice communication control channel. While operating on the analog cellular control channel, the subscriber station monitors for incoming analog cellular voice communications directed to the subscriber station. In a subsequent step, the subscriber station switches back to the CDPD channel before the end of the first time interval.
In yet another aspect of the present invention, a method for operating a wireless subscriber station in a CDPD system includes selecting at the wireless subscriber station a first time interval beginning at the completion of the most recent CDPD communication between a subscriber station and the CDPD system and ending when the subscriber station is expected to enter a CDPD sleep mode. The wireless subscriber station synchronizes with the CDPD communication system so that both the subscriber station and the CDPD system measure a plurality of second time intervals, wherein the second time intervals selected by the CDPD system define durations of time allowed to the subscriber station before registration. The subscriber station monitors for incoming paging signals on a second communication system channel during the first time interval. The subscriber station also monitors for incoming paging signals on the second communication system during a plurality of second time intervals. The subscriber station changes modes to monitor for incoming CDPD communications on the CDPD channel before expiration of the last of the plurality of second time intervals.
In another aspect of the present invention, the objects of the present invention are achieved by a method of effecting handoff of a wireless subscriber station from a first cell to a second cell of a CDPD communication system. A subscriber station contains a cell transfer database pertaining to the first cell, and registers in the second cell of the CDPD system by sending a polling receiver ready (xe2x80x9cRRxe2x80x9d) signal to an MDBS of the second cell. The subscriber station determines a first time interval for a complete Received Signal Strength Indication (RSSI) scan of the second cell. The subscriber station divides the first time interval into a plurality of overlapping sequential time slots. The subscriber station then alternately scans for CDPD activity and analog cellular voice communication activity on alternating time slots for the duration of the first time interval. The information derived from this scanning process creates a second cell transfer database for the second cell. Once this second cell transfer database has been obtained, the first cell transfer database is discarded.
Yet another aspect of the present invention is directed to a wireless subscriber station arranged for communication with a first communication system and a CDPD communication system. The wireless station includes means for requesting a communication on the first communication system and means for requesting a CDPD communication. The wireless subscriber station also includes means for operating on the first communication system while remaining registered on the CDPD communication system.
An additional aspect of the present invention is directed to a method for communicating between a wireless subscriber station and both a first communication system and a CDPD communication system. The method includes the steps of registering the wireless subscriber station with the first communication system and then registering the wireless system with the CDPD communication system. In the final step the subscriber station tunes to a control channel on the first communication system while still presumed by the CDPD communication system to be on the CDPD channel.